Silicone pressure sensitive adhesives, hereinafter also referred to as silicone PSAs, are useful in a variety of applications by virtue of their unique properties, including excellent adhesive and cohesive strength, high tack, very low alpha particle emissions, good moisture resistance, good electrical properties, high ionic purity, and good adhesion to low energy substrates. For example, silicone PSAs are widely used in adhesive tapes, bandages, low-temperature backings, transfer films, and labels. Moreover, silicone PSAs are used in the assembly of automotive parts, toys, electronic circuits, and keyboards.
Addition-curable silicone compositions useful for preparing silicone pressure sensitive adhesives are known in the art. For example, silicone compositions containing an alkenyl-containing polydiorganosiloxane, an organopolysiloxane resin, an organohydrogenpolysiloxane, and a hydrosilylation catalyst are disclosed in U.S. Pat. No. 5,290,885 to Vincent et al.; U.S. Pat. No. 5,366,809 to Schmidt et al.; U.S. Pat. No. 3,983,298 to Hahn et al.; U.S. Pat. No. 5,399,614 to Lin et al.; U.S. Pat. No. 5,100,976 to Hamada et al.; U.S. Pat. No. 5,446,532 to Wengrovius et al.; U.S. Pat. No. 4,774,297 to Murakami et al.; U.S. Pat. No. 4,988,779 to Medford et al.; and U.S. Pat. No. 5,292,586 to Lin et al. The above-cited references disclose adding only small amounts of additional ingredients, including fillers, to the addition-curable silicone compositions. However, these references do not teach the silica filler of the present invention or the concentration of such filler.
However, conventional addition-curable silicone compositions, including the aforementioned, cure to form silicone pressure sensitive adhesives having relatively high coefficients of thermal expansion, rendering them unsatisfactory for certain applications, such as attachment of heat sinks to semiconductor chips and semiconductor chips to substrates. During thermal cycling, stresses develop in the silicone pressure sensitive adhesive due to differences in the coefficients of thermal expansion between the substrate materials and the adhesive. Thermally-induced stresses can weaken or fracture the pressure sensitive bond, limiting the useful service life of a device.
While inorganic fillers such as silica and alumina are typically added to silicone rubber compositions to reduce the coefficient of thermal expansion of cured products, silicone compositions for preparing silicone pressure sensitive adhesives are usually unfilled or contain only low concentrations, for example, less than about 5 percent by weight, of filler. Fillers are generally considered undesirable in the later silicone compositions because moderate to high amounts of filler can cause and increase in viscosity of a given composition and also significantly reduce the tack of the silicone PSA produced therefrom.
Therefore, there is a need for an addition-curable silicone composition that cures to form a silicone pressure sensitive adhesive having a low coefficient of thermal expansion and high tack.